The present invention relates to a heated nozzle unit for the moulding of plastics materials.
Conventionally, an injection nozzle for the moulding of plastics materials comprises a cylindrical, tubular steel core which forms a central longitudinal injection duct for injecting the molten plastics material through one or more injection holes into a moulding cavity of a mould. An electrical resistor is wound as a helix around the tubular core to heat the plastics material which flows through the injection duct and to keep the parts of the nozzle which are affected by the flow of material at a controlled temperature such as to prevent the material from solidifying. The windings of the resistor are usually closer together in the region close to the injection hole, which is closer to the moulding cavity and therefore tends to cool more quickly than the central regions of the nozzle. A capillary thermocouple detects the temperature of the nozzle in the vicinity of the injection-hole. The heat supplied by the resistor tends to accumulate in the central region of the nozzle where higher temperatures are reached than in the region of the injection hole; at times, these higher temperatures are not permissible for the type of plastics material being processed which instead should be kept within a fairly limited temperature range to prevent degradation of the plastics material. Thus, the resistor is activated as soon as the thermocouple detects a temperature below a predetermined minimum value in the region of the injection hole but, even though the temperature of the injection duct in the central regions is acceptable, that temperature rises as a result of the switching-on of the resistor until it exceeds a maximum permissible value for the material.
In these known solutions, spiral resistors of rectangular cross-section are mostly used in order to increase the contact area between the resistor and the tubular nozzle core around which the resistor is wound. However, the contact area naturally constitutes only a fraction of the overall surface of the resistor so that most of the heat generated by the resistor is not actually transmitted to the nozzle but is dissipated into the surrounding mould and is thus lost. In fact the mould is in turn cooled in order to keep the walls of the moulding cavity at as low a temperature as possible in order to speed up the solidification of the molten material and thus shorten the moulding cycles.
In order to dissipate the heat from the central portion of the nozzle and to distribute the heat more uniformly along the injection duct, it has been proposed to incorporate the resistor in a tubular metal diffuser element which is fitted externally on the tubular nozzle core. According to this solution, a channel-like seat is formed in the outer surface of a cylindrical tubular element and the resistor is inserted therein. However, here again, excessive heat dispersal occurs from the outer surface of the resistor towards the surrounding mould; moreover, direct contact (and hence direct transmission of the heat by conduction) is not achieved between the resistor and the tubular nozzle core.